This application is based on Laid-open Japanese Patent Application No. 7-030088, which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a solid-state image pickup device comprising a CCD (Charge Coupled Device), more particularly to a solid-state image pickup device without smears and alloy spikes and a method of manufacturing the same.
2. Description of the Related Art
A CCD based solid-state image pickup device comprises an image pickup element unit and peripheral circuits. The image pickup element unit comprises sensors which convert incident light into signal charges and a charge transfer section which transfer the signal charges to other components. The sensors in the image pickup element unit are arranged in a matrix form. The peripheral circuits are arranged around the image pickup element unit. Both the image pickup elements and the peripheral circuits are formed on a common semiconductor substrate.
In the solid-state image pickup device, a photo shield must be provided so as to cover the image pickup element unit except the sensors in order to prevent smears caused by incident lights upon regions on the image pickup element unit other than the sensors.
The photo shield has the dual film structure including an aluminum film and an anti-reflection film. The aluminum film is used for wiring. The aluminum film has a large reflection rate and reflects light at its sides, ends and back surface. The anti-reflection film is prepared in order to allow the reflected lights from the aluminum film to enter at only the sensors.
However, it is difficult to prevent smears effectively by the anti-reflection film because the reflection rate of the aluminum film is large.
Alloy spiking may occur in the peripheral circuit of the solid-state image pickup device, or the like. Such alloy spiking is caused by direct connection between semiconductor layer and the aluminum wiring film. The semiconductor layer works as a source region and a drain region in a MOS transistor on the circuit. The alloy spikes may deteriorate the connection status between the semiconductor layer and the wiring film.
In a conventional method, a barrier metal layer is formed between the aluminum film and the semiconductor layer in order to avoid alloy spiking.
However, such method brings complication in manufacturing the solid-state image pickup device, because the step of forming the barrier metal layer is separated from the step of forming the anti-reflection film.
Laid-open Japanese patent application No. 7-030088 discloses a technique which avoids the above problem. According to the technique, the anti-reflection film and the barrier metal layer are formed in the same step. Also in this case, the aluminum film is used as the photo shield.
According to the technique more precisely, a titanium nitride film is formed on a semiconductor substrate, and a pure titanium film is deposited onto the titanium nitride film. Then, the substrate is subjected to heat treatment so that the pure titanium film is diffused into the titanium nitride film. The diffused titanium in the titanium nitride film reacts with silicon in a region where contacts are formed on the semiconductor substrate. The reaction causes formation of a titanium silicide layer. The titanium silicide layer is formed at a connection between a diffusion region of the MOS transistor and the titanium nitride layer.
A disadvantage of the above method is insufficiency of smear reduction even though the anti-reflection film is used. This is caused by using aluminum film having a large reflection rate in the photo shield. Moreover, the above method has extra steps for manufacturing the device. That is, a step for reacting titanium with silicon after diffusing pure titanium into the titanium nitride film is required to form the titanium silicide layer. Such titanium silicide layer is necessary because ohmic contact between the diffusion region in the peripheral circuit and the wiring is realized by the titanium nitride film and the titanium silicide layer. Furthermore, large contact resistance may appear.
The present invention has been made in consideration of the above. It is an object of the present invention to provide a solid-state image pickup device and a method of manufacturing the same which accomplish smear reduction.
It is another object of the present invention to provide a solid-state image pickup device and a method of manufacturing the same which realize excellent ohmic contact without alloy spiking in a simple way.
To accomplish the above objects, a solid-state image pickup device according to a first aspect of the present invention comprises:
an image pickup element unit, formed on a semiconductor substrate, comprising matrix formed sensors for converting incident light into charges and a photo shield for covering regions other than said sensors; and
peripheral circuits arranged around said image pickup element unit formed on said semiconductor substrate comprising contacts made of a refractory metal film which is used in said photo shield.
According to this structure, since the solid-state image pickup device uses a refractory metal film (high melting point metal film) as a photo shield, it reduces smears more effectively than a solid-state image pickup device using an aluminum film as the photo shield.
Said photo shield and said contacts may be made of tungsten, or a laminated film comprising a titanium nitride film and a titanium film, and a tungsten film which is formed on said laminated film.
Said contacts and said semiconductor substrate may be connected to each other directly. A silicide layer of refractory metal may be formed between said contacts and said semiconductor substrate.
An aluminum film may be formed on said refractory metal layer of said contacts.
A wiring film comprising any one of a tungsten film, an aluminum film, or a laminated film of said tungsten film and said aluminum film is formed on regions other than said contacts.
A method of manufacturing a solid-state image pickup device according to a second aspect of the present invention comprises:
preparing a semiconductor substrate on which an image pickup element unit comprising sensors for converting incident light into charges, a charge transfer unit for transferring the charges from said sensors, and peripheral circuits arranged around said image pickup element unit;
forming a refractory metal film, on a whole surface of said semiconductor substrate after forming contact holes at predetermined positions on said peripheral circuits; and
removing said refractory metal film on said sensors in said image pickup element unit and on regions other than contact holes in said peripheral circuits.
According to this structure, since the solid-state image pickup device uses a refractory metal film as a photo shield, it reduces smears more effectively than a solid-state image pickup device using an aluminum film as the photo shield.
Moreover, the manufacturing process is simplified because the photo shield and the contacts are formed in the same step.
A refractory metal film made of tungsten may be formed on the whole surface of said semiconductor substrate.
The method may comprise:
forming a wiring film comprising any one of a tungsten film, an aluminum film or a laminated film of said tungsten film and said aluminum film on regions other than said contacts, or
forming a laminated film of a titanium nitride film and a titanium film on said prepared semiconductor substrate; and
forming a tungsten film on said laminated film.
The method may further comprise:
transforming conjunction regions between said refractory metal film in said contact holes and said semiconductor substrate to silicide by heat treatment applied to said semiconductor substrate.
Said semiconductor substrate may be subjected to the heat treatment in an inactive atmosphere whose temperature is equal to or greater than 450 degrees Celsius and equal to or less than 600 degrees Celsius for 1 to 30 minutes to form the silicide.
The method may comprise:
forming an aluminum film at least on said contact holes in said peripheral circuits after the silicide is formed.